Shaped articles comprising high density polycrystalline silicon carbide are well known. They are characterised by excellent physical properties such as high resistance to thermal shock, abrasion and oxidation together with high levels of strength and thermal conductivity. It is this combination of properties which makes silicon carbide materials leading candidates for engineering applications. However, the production of satisfactory high density materials has been fraught with difficulties.
Early workers (eg. Alliegro, Coffin and Tinkepaugh J. Amer. Ceram. Soc., 39 11! 386-89 1956!) showed that silicon carbide could be hot pressed to high density bodies with the aid of sintering aids such as aluminium and iron, and aluminium plus one of the metals zirconium, boron or iron. It was further disclosed that for the hot pressing of silicon carbide, magnesium additions and magnesium and aluminium additions were ineffective, and impaired the densification process as compared to a control sample of silicon carbide, hot pressed under identical conditions with no additives or additions. Lange (J. Mater. Sci. (10 1975!314-320) disclosed the hot pressing of silicon carbide using alumina as the densification aid. The limitations of hot pressing for the attainment of dense bodies are well known.
The selection of suitable densification aids for the sintering of silicon carbide has been considered by Negita (J. Am. Ceram. Soc. 6912!C308-10 1986!). Using thermodynamic arguments it was found that metal additives such as boron, aluminium, iron, nickel and cobalt could be effective densification aids. Using these principles, alumina, beryllia, yttria, hafnia and rare earth oxides are considered to be potential densification aids as they do not decompose silicon carbide during sintering. Metal oxides including zirconia, calcia, magnesia are not considered suitable as they tend to decompose silicon carbide to metallic silicon. In addition the use of carbon with metal oxide additions was reported to be beneficial for oxides such as alumina, beryllia, ytrria, rare earth oxides, calcia, zirconia, and hafnia. It is stated that the carbon is added to react with the said oxides to form the corresponding metal carbide and silicon metal. The formation of the metal carbides was seen as desirable. In the process according to the present invention, the formation of such metal carbides was not observed. Furthermore, in contrast to the work of Negita, in the current work it has been found that the reaction of carbon with the metal oxide densification aids is undesirable and impairs the densification of the bodies. This indicates that the role of carbon in the present work is different to that proposed by Negita and others. In addition, given the unstable nature of metal carbide phases, for some even in air at room temperature, the formation of such phases is seen as undesirable and are avoided in the present invention. This aspect will be discussed in greater detail for the calcia system.
The work of Cutler and Miller (U.S. Pat. No. 4,141,740) describes a process for a refractory product based on silicon carbide containing at least 1% by weight of aluminium nitride and at least 1% by weight aluminium oxycarbide. The presence of metal impurities (other than aluminium and silicon) were seen to be detrimental to the process and are limited to 0.1 percent by weight or less. No indication was given as to the properties of such bodies and the ease at which they can be made into dense bodies with desirable physical properties and the commercial utility of the process. Further work in this system was described by Virkar et al in International Patent application WO87/01693, where the pressureless sintering of silicon carbide-aluminium nitride-aluminium oxycarbide containing materials was described. A major drawback of the process as disclosed is that the materials must be heated at very rapid rates to minimize volatilisation of the active densification species. This could pose problems for the production of large parts in which differential sintering as a result of thermal gradients can lead to distortion and ultimately to micro cracking due to thermal stresses inevitably present as a result of the described firing cycles. This would make the maintenance of the desired physical properties difficult. In addition, the undesirable presence of aluminium oxycarbides in the final body may prove difficult to avoid.
In Suzuki et al. (U.S. Pat. No. 4,354,991) the use of aluminium oxide to densify silicon carbide is described. In the process as described the use of non-oxidative atmospheres is taught. These include nitrogen, carbon monoxide, helium and argon. It is taught that argon or helium are preferable and that the atmosphere should preferably contain aluminium, silicon or carbon. In one method, it is proposed that mixtures of these gases be fed into the reaction chamber with a carrier gas such as nitrogen, argon and helium. In another method, the use of a powder bed or sintered product capable of generating the gases around the silicon carbide article to be densified was disclosed. It was a teaching of the document that it is unnecessary to remove the silica present on the surface of the silicon carbide. In fact it was stated that it is feasible to add silica as a raw material. This is in contrast to the present invention where the presence of this phase has been found to exert a deleterious effect on the densification behaviour at high temperatures and will be explained later on. The fired bulk densities obtained were inferior to those achieved by the present invention. In addition, the sintering times were much longer. For a continuous process for the densification of bodies, the significantly longer reaction times for densification would result in lower production rates.
In the work of Fuentes U.S. Pat. No. 4,876,226 the use of alumina and calcia as densification aids for silicon carbide was disclosed. It was a requirement of the invention to form liquid phases comprising aluminium oxycarbides at the sintering temperatures to promote densification. The addition of calcia was to increase the amount of the aluminium oxycarbide liquids and enhance densification. It was further disclosed that the addition of free carbon is preferred. It is believed that in the system described by Fuentes, the addition of free carbon is to react with aluminium containing phases to produce or further enhance the formation of the oxycarbide phases which are a requirement for the process. The level of free carbon additions were prefereably greater than 0.4% by weight. No indication was given to an upper limit for the carbon addition. This is in contrast to the teachings of the present invention, where the reaction of carbon with the aluminate phase is believed to be detrimental to the densification process. It is claimed that the technique excludes the use of rare earths but no reference was given for their deletion. Indeed, in the work of Omori et al (U.S. Pat. No. 4,569,921) the use of calcia and precursors for the oxides of aluminium and rare earth elements for the pressureless sintering of silicon carbide was disclosed with excellent results. In addition, it has been reported by Foster et al (J. Am. Ceram. Soc. 391!-111956!) that aluminium carbide and aluminium oxycarbide, the latter at least required for the process as outlined, are very unstable towards both moisture and oxygen. They taught that these materials should not be used in applications where these species are likely to be encountered. In the process as disclosed, such aluminium oxycarbide species are a key element of the process. The presence of such species is expected to greatly degrade the performance and severely limit the suitability of the said materials. In the present invention, aluminium oxycarbides, such as Al.sub.4 O.sub.4 C and Al.sub.2 OC, have not been observed and their presence is not a prerequisite for the process disclosed herein. Thus the process and product as disclosed herein overcomes significant disadvantages of the process as disclosed by Fuentes.
The use of rare earths and alumina as sintering assists for silicon carbide has been disclosed (eg see Mulla and Krstic Bull. Amer. Ceram. Soc. 703!439-443 1991!). In order to obtain high density bodies, the components had to be encapsulated in closed threaded graphite crucibles sealed with graphite foil. It was revealed that bodies could be produced with over 95% of theoretical density and weight losses of less than 1 percent. When the same experiments were carried out without encapsulation, the resulting bodies obtained less than 80 percent of theoretical density and weight losses up to 20 percent were recorded. Culter and Jackson (pp 309-318 in Ceramics Materials and Components for Engines, Proceedings of the Third International Symposium, Las Vegas Nev. 1988) also disclosed the use of yttria and alumina for the sintering of silicon carbide. Although high density bodies could be produced, the recorded weight losses were high and increased with increasing temperature. The decomposition reactions between the sintering assists and the silicon carbide were cited as a major problem. As in the case of Mulla and Krstic only very short times were used, typically of 5 minutes duration at the maximum temperature. The requirement to subject samples to minimum times at high temperatures is considered difficult to carry out on a commercial scale especially for the manufacture of larger components or where large furnace loads are used.
This can lead to large thermal gradients giving rise to problems such as differential sintering, leading to distortion of the fired bodies. This would greatly reduce the application utility of the process.